Examples of existing eyedropper, especially those for applying a medical fluid such as medical eyedrops, which are in wide use, are what is called a three-piece type eyedropper including a main container body in the form of a hollow cylinder, with an instilling tube attached thereto, and with a cap mounted on the main container body to make three components forming a whole eyedropper, and, as shown in FIG. 11, what is called a bottle pack eyedropper X which may be an integral molding type container A, with an instilling tube 6 and a main container body 10 formed integrally by blow molding or vacuum forming technique, and with a cap B screwed or fitted to the container A. It is also known that the instilling tube 6 has a stopper with an instilling opening 6a mounted on a tip end of the tube (see FIGS. 1 and 2 of Japanese Utility Model Publication No. 39-11991). As a material of the eyedropper X of this type, a soft thermoplastic resin is used because it is easy to form and so on.
When the medical fluid contained in the eyedrops container A is applied with this type of eyedropper X, a barrel 2 of the eyedrops container A (main container body 10) is held by tips of two digits to keep an applying posture where the instilling opening 6a of the eyedrops container A faces the eye to receive the fluid. The barrel 2 is pressed toward an axis of the container while maintaining the applying posture, whereby the medical fluid is distilled and applied from the instilling opening 6a. 
Since the medical fluid, especially medical eyedrops are applied directly to the eye which is an especially sensitive organ in the human body, it is strictly required to maintain them in aseptic condition until instillation. Therefore, an aseptic filling is required to be carried out in an aseptic room meeting strict conditions after sterilizing each component of the above-noted eyedropper with steam sterilization or with EO gas sterilization, and after sterilizing the medial fluid with sterilization by filtration using a membrane filter and so on.
With the eyedrops container of the above conventional eyedropper (the integral molding type container, for example), the eyedrops container is filled with the medical fluid in aseptic condition and then the cap is mounted on the eyedrops container to seal the container. Thus, the medical fluid contained in the eyedrops container can be prevented from directly contacting ambient air, as a result of which the medical fluid can usually be maintained in aseptic condition until the cap is released for instillation.
However, as the medical fluid is distilled and applied from the instilling opening after the eyedrops container is opened, ambient air flows into the eyedrops container from the instilling opening in a volume corresponding to the medical fluid applied. At this time, the aseptic condition within the eyedrops container cannot be secured because of germs present in the ambient air or the like, which might pollute the interior of the eyedrops container (interior pollution of the eyedrops container after it is opened).
In order to avoid such interior pollution, a method of filling the eyedrops container with the medical fluid added with an antiseptic solution in advance is in wide use (antiseptic treatment of the medical fluid).
On the other hand, attempts have been made to prevent the germs present in ambient air being taken into the eyedrops container. The present invention has been made from this point of view.
Incidentally, a completely integrally formed and sealed eyedrops container is in wide use, which container dispenses with the instilling opening for discharging the medical fluid in order to avoid contact with ambient air. It is desirable to provide a cap advantageously mounted on such an eyedrops container in a sealed condition.
Therefore, the object of the present invention is to provide a cap for mounting on a container in a sealed condition containing a fluid and capable of preventing the fluid from contacting ambient air before the container is opened and preventing pollution of the container interior after it is opened.